1. Field of the Invention
The present invention relates to an image processing apparatus and a method for generating a color conversion table used for color-separation-limited printing by the image processing apparatus.
2. Description of the Related Art
Writing with a pen using a colorant in a color other than black on a part, to be highlighted, to be corrected, etc., of a computer-aided designing (CAD) drawing or a document that is printed on paper is sometimes performed. When a document on which such writing with the pen is performed is scanned with a scanner or the like to form electronic data thereof and to print it with a printer or the like, image processing is sometimes applied to make the part, on which writing with the pen is performed, stand out.
For such an application, a method called color-separation-limited printing is known in which color within a given hue range among color in information of full-color input images is output using two different color materials (for example, magenta toner and black toner), number of which are less than number of color materials used in ordinary full-color printing, and color outside of the given hue range is output using a single toner (for example, black toner) from the viewpoint of toner saving.
In conventional color-separation-limited printing, to reduce difference in printed color compared with ordinary printing and to share common processes with the ordinary printing, the same color conversion and black generation parameters as the ordinary printing are used and the same image processing as the ordinary printing is performed, and a some number of resultant color separation images are forcibly deleted. As a result, missing color or excessive separation in gradient (too small separation in gradient) might be formed particularly in color from a high-saturation part to low-lightness part.
For example, in the technology disclosed in Japanese Patent No. 4217536, color to be used in printing is specified, and an input color image signal is converted into a multi-valued signal representing the specified color by making two of the CMY components have the same value. The black generation process is then performed on the multi-valued signal, thereby outputting a multi-valued CMYK signal. However, this technology does not solve the problem of formation of visible excessive separation in gradient formed in color from the high-saturation part to low-lightness part.
There is a need to provide an image processing apparatus that can prevent missing color in color-separation-limited printing, while maintaining the gradation characteristics and visibility in color from the high-saturation part to the low-lightness part, and a method for generating a color conversion table used for color-separation-limited printing by the image processing apparatus.